Nociceptors are primary sensory afferent (C and Aδ fibers) neurons that are activated by a wide variety of noxious stimuli including chemical, mechanical, thermal, and proton (pH<6) modalities. The lipophillic vanilloid, capsaicin, activates primary sensory fibers via a specific cell surface capsaicin receptor, cloned as the transient receptor potential vanilloid-1 (TRPV1). TRPV1 is also known as vanilloid receptor-1 (VR1). The intradermal administration of capsaicin is characterized by an initial burning or hot sensation followed by a prolonged period of analgesia. The analgesic component of the TRPV1 receptor activation is thought to be mediated by a capsaicin-induced desensitization of the primary sensory afferent terminal. Thus, the long lasting anti-nociceptive effect of capsaicin has prompted the clinical use of capsaicin analogs as analgesic agents. Further, capsazepine, a capsaicin receptor antagonist can reduce inflammation-induced hyperalgesia in animal models. TRPV1 receptors are also localized on sensory afferents, which innervate the bladder. Capsaicin or resiniferatoxin has been shown to ameliorate incontinence symptoms upon injection into the bladder.
The TRPV1 receptor has been called a “polymodal detector” of noxious stimuli since it can be activated in several ways. The receptor channel is activated by capsaicin and other vanilloids, and thus is classified as a ligand-gated ion channel. The TRPV1 receptor activation by capsaicin can be blocked by the competitive TRPV1 receptor antagonist, capsazepine. The channel can also be activated by protons. Under mildly acidic conditions (pH 6-7), the affinity of capsaicin for the receptor is increased, whereas at pH<6, direct activation of the channel occurs. In addition, when membrane temperature reaches 43° C., the channel is opened. Thus heat can directly gate the channel in the absence of ligand. The capsaicin analog, capsazepine, which is a competitive antagonist of capsaicin, blocks activation of the channel in response to capsaicin, acid, or heat.
The channel is a nonspecific cation conductor. Both extracellular sodium and calcium enter through the channel pore, resulting in cell membrane depolarization. This depolarization increases neuronal excitability, leading to action potential firing and transmission of a noxious nerve impulse to the spinal cord. In addition, depolarization of the peripheral terminal can lead to release of inflammatory peptides such as, but not limited to, substance P and CGRP, leading to enhanced peripheral sensitization of tissue.
Recently, two groups have reported the generation of a “knock-out” mouse lacking the TRPV1 receptor. Electrophysiological studies of sensory neurons (dorsal root ganglia) from these animals revealed a marked absence of responses evoked by noxious stimuli including capsaicin, heat, and reduced pH. These animals did not display any overt signs of behavioral impairment and showed no differences in responses to acute non-noxious thermal and mechanical stimulation relative to wild-type mice. The TRPV1 (−/−) mice also did not show reduced sensitivity to nerve injury-induced mechanical or thermal nociception. However, the TRPV1 knock-out mice were insensitive to the noxious effects of intradermal capsaicin, exposure to intense heat (50-55° C.), and failed to develop thermal hyperalgesia following the intradermal administration of carrageenan.
U.S. Pat. No. 7,129,235 describes compounds of formula (a) that are vanilloid receptor antagonists and are useful in treating pain
wherein A is pyridinyl, phenyl, thiazolyl, oxazolyl, imidazolyl, etc., R11-14 are hydrogen, alkoxy, alkyl, or hydroxy, R6, R7, and R8 are hydrogen, alkyl, haloalkyl, and the like.
U.S. Application publication No. 2006/0128755 discloses compounds of formulae (b) and (c) that exhibit anti-inflammatory and analgesic activities
wherein Xa is Cl or CF3, and Xb is N or CH.
In addition, a cyclo(hetero)alkenyl compound of formula (d) has been disclosed in publication
wherein V is N or CH, R3 is C1-10 alkyl, halo, and the like, m is 0 or 1, Ar1 is pyridinyl, pyrimidinyl, pyrazinyl, pyridazinyl, or thiadiazolyl, X is O or S, Ar2 is phenyl, pyridinyl, cyclohexyl, cyclohexenyl, and the like and are reported as vanilloid receptor inhibitors.
However, none of these compounds has the structural characteristics of the compounds of the present invention wherein the compounds of the invention contain a chiral carbon center in the tetrahydropyridine ring and possess unexpected efficacy as TRPV1 antagonists.